For example, JP4076433B2 and JP2007-285281A teach a technique of limiting emissions at an internal combustion engine of a direct injection type. Specifically, nitrogen-rich air (low-oxygen-concentration gas), which has a high content of nitrogen and is produced by removing a portion of oxygen from the atmospheric air, is supplied to a combustion chamber of the internal combustion engine. In this way, combustion of fuel at an outer peripheral portion of fuel mist, which is directly injected from a fuel injection valve into the combustion chamber, is slowed to reduce the combustion temperature and thereby to limit generation of the nitrogen oxide (NOx).
Although the above technique can limit the generation of the nitrogen oxide (NOx), the above technique may not appropriately limit generation of particulate matter (smoke). This is because of combustion of a high-fuel-concentration area of the fuel mist, which has a high fuel concentration, under shortage of the oxygen.